Rhodium can be an active metal component of catalyst systems used as either a reduction catalyst or a three way catalyst. Rhodium is used because of its known selectivity to oxidize carbon monoxide by nitrogen oxide in the presence of stoichiometricly balanced gas mixtures or slightly oxidizing gas mixtures. Under these conditions most base metal oxide catalyst and noble metal catalysts oxidize carbon monoxide preferentially by the oxygen available in the gas mixture and therefore do not treat the nitrogen oxides present.
The supply of rhodium in the world is generally limited by the mine ratio at which it occurs. Throughout the world, the general mine ratio of platinum to rhodium is about 17 to 1. In other words, for every 17 ounces of platinum obtained from a mine, 1 ounce of rhodium is obtained from that same mine during the process of extracting platinum. This ratio had been quoted in the recent past as 19.1 and still today the ratio of platinum to rhodium does vary from mine to mine. Therefore, in any large scale application of a three way catalyst to automotive exhaust gas emissions treatment which would contain rhodium, the rhodium content of the catalyst will be limited to its mine ratio because no one would discard the platinum mined with the rhodium. Also, if only rhodium was used as a catalyst, it soon would be depleted.
If rhodium is used in the ratio of platinum to rhodium of 17/1 on a three way catalyst, we have found that there is formation of ammonia when exhaust gas mixtures from an internal combustion engine which are overall reducing in composition are passed over the catalyst. This occurs because platinum has a poor selectivity for conversion of oxides of nitrogen to nitrogen. A large fraction of nitrogen oxides chemisorb which react on platinum are converted to ammonia instead of nitrogen.
Rhodium, on the other hand, is particularly selective for the reduction of oxides of nitrogen to nitrogen. Thus, if rhodium is made to be the principal catalyst material for conversion of oxides of nitrogen to nitrogen, less ammonia is produced.
It is the principal object of this invention to provide a catalyst system and method of making the same which produces a rhodium containing catalyst in which the amount of rhodium which is actively available for use as a catalyst is enriched over that which is available in the as manufactured catalyst system.